The present invention relates to a system for the utilization of low-grade heat such as solar energy or the waste heat of a power generating plant by utilizing the large variation of the sorption capacity of molecular sieve zeolite, and other sorportion materials, such as activated carbon and silica gel, with variations of temperature. In particular, the system relates to a system which converts small variations in absolute temperature to relatively large variations of gas pressure which is utilized to produce mechanical or electrical energy or cooling in refrigeration.
One of the primary difficulties which hinders the utilization of solar energy for heat and cooling purposes is its low energy density (less than 1.5 kilowatt per square meter) of solar energy on earth. The temperature differentials obtained with solar energy collectors are small and even when solar concentrators are used, temperatures above 200.degree. - 300.degree. centigrade require sophisticated sun-following techniques. Thus, a need exists to develop methods for efficient energy conversion at small temperature differentials, say between 30.degree. - 100.degree. centigrade. Materials exist which will permit the design of such systems, especially to satisfy the needs for home cooling and air-conditioning. The output of such systems increases as the solar load increases and therefore the higher needs for cooling automatically are met by the higher output of such systems. Although the primary objective of this invention is to provide an alternative approach to solar energy cooling and air-conditioning of buildings, the system may also be utilized for the development of large-scale systems capable of operating from waste heat power plants and other thermal polluters thereby reducing the pollution and converting it to useful energy.
Those skilled in the art understand that due to the low temperature differentials obtainable with solar energy, Carnot efficiency of any system using the normal expansion of gases is of necessity quite low. For this reason, most solar energy refrigeration systems have concentrated on the old, well proven absorption refrigeration cycle based on the change of the solubility of a gas in a liquid with temperature. Inasmuch as this process is thermally activated, its dependence on temperature is exponential which permits large changes of gas pressure for small changes in absolute temperature. This process has received new impetus by commercial use of systems other than the ammonia-water used in early gas refrigerators. For example, at Kennedy Airport, New York City, an air conditioning system is provided which utilizes lithium bromide and water as working fluids.
Molecular sieve zeolites comprise a solid material capable of absorbing large quantities of different gases and having even stronger temperature dependence than the presently used exponential one. These materials lend themselves to a unique design which utilizes solid materials and diffusion through them to provide a solar refrigeration system of high conversion efficiency without moving parts and therefore capable of long life and reliability.
The amount of absorbed gas in a molecular sieve is represented by the equation EQU a = a.sub.o.sbsb.2 .theta..sub.2 + a.sub.o.sbsb.n .theta..sub.n
where a.sub.o is the limiting adsorption value of the gas and .theta..sub.n = exp[(RTln (p.sub.s /p )/E.sub.n ].sup.n and n is an integer between 2 and 5. R is the universal gas constant; p.sub.s is the limiting saturation pressure; p is the actual pressure; and E.sub.n is the activation energy, which is on the order of a few kilocalories per mole. In this connection, reference is made to M. Dubin and V. Astakhov, "Description of Adsorption Equilibria of Vapors on Zeolites Over Wide Ranges of Temperature and Pressure," Second International Conference on Molecular Sieve Zeolites, Sept. 8-11, 1970, Worcester Polytechnic Institute, Worcester, Mass., pp. 155-166.
In view of the foregoing, it will be understood that the dependence of gas absorption on temperature is at least exponential with a square of temperature and may go as high as to be exponential to the 5th power of the temperature. (For example, acetylene on zeolite NaA).